In order to explain the background of the invention, reference will be particularly made to FIG. 2 which shows a prior art base drive circuit for switching a bipolar Darlington power transistor at a high speed:
The bipolar Darlington power transistor 2 which is driven by the current from the base drive circuit 1 is constituted by NPN bipolar transistors Q1 and Q2 in Darlington connection, a diode 3 and a resistor 4 for speeding up the operation of the transistors Q1 and Q2, and a flywheel diode 5 for absorbing the commutation current at the turn-off. The anode of the diode 3 is connected to the emitter of the transistor Q1, and the cathode of the diode 3 is connected to the base of the transistor Q1. The resistor 4 is provided connected between the base and the emitter of the transistor Q2. The one end of the resistor 4 is connected to the anode of the diode 3 and the base of the transistor Q2 (the emitter of the transistor Q1). The cathode of the flywheel diode 5 is connected to the collector of the transistor Q2, and the anode of the flywheel diode 5 is connected to the emitter of the transistor Q2.
A driver circuit for driving the bipolar Darlington power transistor 2 at a high speed and at a high frequency is constituted by a high speed switching diode 7 which applies a regular direction base current from the base drive circuit 1 to the base of the transistor Q1 of the Darlington power transistor 2, a Baker clamp diode 6 for applying a regular direction extra base current to the collector of the transistor 2, and a high speed switching diode 8 for discharging the reverse direction base current from the transistor 2. The regular direction voltage drop of the Baker clamp diode 6 and that of the high speed switching diode 7 are preferably made equal to each other.
This circuit is operated as follows:
Firstly, the operation at the turning-on is described. The transistor 2 is turned on by the regular direction base current given through the high speed switching diode 7 from the base drive circuit 1, and the collector current begins to be flow. The base-collector path of the transistor 2 becomes regular biased because the base-emitter path thereof is already saturated. The sum of the regular bias voltage between the base and collector thereof and the regular voltage drop of the high speed switching diode 7 becomes larger than the regular voltage drop of the Baker clamp diode 6, and the diode 6 is turned on, whereby the extra base current is given to the collector of the transistor 2 through the Baker diode 6. As a result, even if the base current increases, the transistor 2 always operates in an unsaturated or quasi-saturated region.
At the time of turn-off a reverse direction base current is given to the base of the transistor 2 through the diode 8. The transistor 2 is operated in an unsaturated or quasi-saturated region, and the storage time is almost zero, thereby enabling a high speed response. The reverse direction current which occurs in a case where an inductive load is absorbed through the flywheel diode 5, or the resistor 4 and the diode 3.
In this way, the prior art bipolar Darlington power transistor under such a construction operates at a high speed and a high frequency.
It is a requirement that the Baker clamp diode 6 used in the circuit of FIG. 2 have the same blocking voltage rating as the voltage rating between the collector and base of the Darlington power transistor 2. Furthermore, it is required for the Baker clamp diode 6 and the high speed switching diode 7 to have equal regular voltage drops. However, when the blocking voltage rating of the power transistor 2 becomes large, it is quite difficult to obtain diodes which satisfy both of these requirements.
Furthermore, when the current flowing through the base of the transistor 2 is not too large, there is scarcely an advantage in making the transistor 2 operate in an unsaturated region by bypassing the extra base current with the Baker diode 6.
Another prior art bipolar Darlington power transistor is disclosed in an article "Storage times and fall times of Darlington transistors for various method of turn-off", by Merle Morozowich, PCl April, 1984 Proceedings. This article discloses that the storage and fall times directly affect the frequency of operation and that the fall time directly affects the switching power dissipation. This study will enable the design engineer to predict storage and fall times and assist in choosing a safe frequency of operation at minimum power dissipation for all high voltage Darlington power transistors.